(current-seconds) returns the current time in seconds. This time
is always an exact integer based on a platform-specific starting date
(with a platform-specific minimum and maximum value).

The value of (current-seconds) increases as time passes
(increasing by 1 for each second that passes). The current time in
seconds can be compared with a time returned by
file-or-directory-modify-seconds (see section 11.3.3).

(seconds->datesecs-n) takes secs-n, a platform-specific
time in seconds (an exact integer) returned by
current-seconds or
file-or-directory-modify-seconds, and returns an instance
of the date structure type, as described below. If
secs-n is too small or large, the exn:fail exception is raised.

The value returned by current-seconds or
file-or-directory-modify-seconds is not portable among
platforms. Convert a time in seconds using seconds->date when
portability is needed.

The date
structure type has the following fields:

second : 0 to 61 (60 and 61 are for unusual leap-seconds)

minute : 0 to 59

hour : 0 to 23

day : 1 to 31

month : 1 to 12

year : e.g., 1996

week-day : 0 (Sunday) to 6 (Saturday)

year-day : 0 to 365 (364 in non-leap years)

dst? : #t (daylight savings time) or #f

time-zone-offset : the number of seconds east of
GMT for this time zone (e.g., Pacific Standard Time
is -28800), an exact integer
52

The date structure type is transparent to all inspectors (see
section 4.5).

(current-milliseconds) returns the current ``time'' in fixnum
milliseconds (possibly negative). This time is based on a
platform-specific starting date or on the machine's startup
time. Since the result is a fixnum, the value increases only over a
limited (though reasonably long) time.

(current-inexact-milliseconds) returns the current ``time'' in
positive milliseconds, not necessarily an integer. This time is based
on a platform-specific starting date or on the machine's startup
time, but it never decreases (until the machine is turned off).

(current-process-milliseconds) returns the amount of processor
time in fixnum milliseconds that has been consumed by the MzScheme
process on the underlying operating system. (Under Unix and Mac OS X, this
includes both user and system time.) The precision of the result is
platform-specific, and since the result is a fixnum, the value
increases only over a limited (though reasonably long) time.

(current-gc-milliseconds) returns the amount of processor time in
fixnum milliseconds that has been consumed by MzScheme's garbage
collection so far. This time is a portion of the time reported by
(current-process-milliseconds).

The time-apply procedure collects timing information for a
procedure application:

(time-applyproc arg-list) invokes the procedure proc
with the arguments in arg-list. Four values are returned: a
list containing the result(s) of applying proc, the number of
milliseconds of CPU time required to obtain this result, the number
of ``real'' milliseconds required for the result, and the number of
milliseconds of CPU time (included in the first result) spent on
garbage collection.

The reliability of the timing numbers depends on the platform; see
section 15.1.2 for more information on time accounting. If
multiple MzScheme threads are running, then the reported time may
include work performed by other threads.

The time syntactic form reports timing information
directly to the current output port:

(time expr) times the evaluation of expr,
printing timing information to the current output port. The result of
the time expression is the result of expr.

(subprocessstdout-output-port stdin-input-port
stderr-output-port command-path arg-string···) creates a new process in
the underlying operating system to execute command-path
asynchronously. The command-path argument is a path to a
program executable, and the arg-strings are command-line
arguments for the program. Under Unix and Mac OS X, command-line
arguments are passed as byte strings using the current locale's
encoding (see section 1.2.3).

Under Windows, the first arg-string can be 'exact,
which triggers a Windows-specific hack: the second arg-string
is used exactly as the command-line for the subprocess, and no
additional arg-strings can be supplied. Otherwise, a
command-line string is constructed from command-path and
arg-string so that a typical Windows console application can
parse it back to an array of arguments.53 If 'exact is
provided on a non-Windows platform, the
exn:fail:contract exception is raised.

Unless it is #f, stdout-output-port is used for the
launched process's standard output, stdin-input-port is used
for the process's standard input, and stderr-output-port is
used for the process's standard error. All provided ports must be
file-stream ports. Any of the ports can be #f, in which case
a system pipe is created and returned by subprocess. For
each port that is provided, no pipe is created and the corresponding
returned value is #f.

The subprocess procedure returns four values:

a subprocess value representing the created process;

an input port piped from the process's standard output, or
#f if stdout-output-port was a port;

an output port piped to the process standard input, or
#f if stdin-input-port was a port;

an input port piped from the process's standard error, or
#f if stderr-output-port was a port.

Important: All ports returned from subprocess must be
explicitly closed with close-input-port and
close-output-port.

The returned ports are file-stream ports (see section 11.1.6),
and they are placed into the management of the current custodian (see
section 9.2). The exn:fail exception is raised when a low-level error
prevents the spawning of a process or the creation of operating
system pipes for process communication.

A subprocess value can be used to obtain further information about the
process:

(subprocess-waitsubprocess) blocks until the process
terminates, then returns void.

(subprocess-statussubprocess) returns
'running if the process is still running, or its exit
code otherwise. The exit code is an exact integer, and 0
typically indicates success. If the process terminated due to a fault
or signal, the exit code is non-zero.

(subprocess-killsubprocess force?) terminates the
subprocess if force? is true and if the process still
running, then returns void. If an error occurs during termination,
the exn:fail exception is raised.

If force? is #f under Unix and Mac OS X, the subprocess is
sent an interrupt signal instead of a kill signal (and the subprocess
might handle the signal without terminating). Under Windows, no
action is taken when force? is #f.

(subprocess-pidsubprocess) returns the operating system's
numerical ID for the process (if any), valid only as long as the
process is running. The ID is an exact integer.

opens the PLT Scheme home page in a browser window. For platforms
other than Windows, the exn:fail:unsupported exception is raised.

The verb-string can be #f, in which case the operating
system will use a default verb. Common verbs include "open",
"edit", "find", "explore", and
"print".

The target-string is the target for the action, usually a
filename path. The file could be executable, or it could be a file
with a recognized extension that can be handled by an installed
application.

The parameters-string argument is passed on to the system to
perform the action. For example, in the case of opening an
executable, the parameters-string is used as the command line
(after the executable name).

The dir-path is used as the current directory when performing
the action.

The show-mode-symbol sets the display mode for a Window
affected by the action. It must be one of the following symbols; the
description of each symbol's meaning is taken from the Windows API
documentation.

'sw_hide or 'SW_HIDE -- Hides the
window and activates another window.

'sw_maximize or 'SW_MAXIMIZE --
Maximizes the window.

'sw_minimize or 'SW_MINIMIZE --
Minimizes the window and activates the next top-level window in the
z-order.

'sw_restore or 'SW_RESTORE --
Activates and displays the window. If the window is minimized or
maximized, Windows restores it to its original size and position.

'sw_show or 'SW_SHOW -- Activates
the window and displays it in its current size and position.

'sw_showdefault or 'SW_SHOWDEFAULT
-- Uses a default.

'sw_showmaximized
or 'SW_SHOWMAXIMIZED -- Activates the window and
displays it as a maximized window.

'sw_showminimized
or 'SW_SHOWMINIMIZED -- Activates the window and
displays it as a minimized window.

'sw_showminnoactive
or 'SW_SHOWMINNOACTIVE -- Displays the window as a
minimized window. The active window remains active.

'sw_showna or 'SW_SHOWNA --
Displays the window in its current state. The active window remains
active.

'sw_shownoactivate
or 'SW_SHOWNOACTIVATE -- Displays a window in its most
recent size and position. The active window remains active.

'sw_shownormal or 'SW_SHOWNORMAL
-- Activates and displays a window. If the window is minimized or
maximized, Windows restores it to its original size and position.

If the action fails, the exn:fail exception is raised. If the action succeeds,
the result is #f. In future versions of MzScheme, the
result may be a subprocess value (see section 15.2) if the
operating system did returns a process handle (but if a subprocess
value is returned, its process ID will be 0 instead of the
real process ID).

(getenvname-string) gets the value of an operating system
environment variable. The name-string argument cannot contain a
null character; if an environment variable named by name-string
exists, its value is returned (as a string); otherwise, #f is
returned.

(putenvname-string value-string) sets the value of an operating
system environment variable. The name-string and
value-string arguments are strings that cannot contain a null
character; the environment variable named by name-string is set
to value-string. The return value is #t if the
assignment succeeds, #f otherwise.

(system-type[mode]) returns information about the operating
system, build mode, or machine for a running MzScheme. The mode
argument must be either 'os (the default),
'link, or 'machine. In 'os mode,
the possible symbol results are:

'unix

'windows

'macosx

In 'link mode, the possible symbol results are:

'static (Unix)

'shared (Unix)

'dll (Windows)

'framework (Mac OS X)

(Future ports of MzScheme may expand the list of system and link symbol
results.)
In 'machine mode, then the result is a string, which contains
further details about the current machine in a platform-specific
format.

(system-language+country) returns a string to identify the
current user's language and country. Under Unix and Mac OS X, the
string is five characters: two lowercase ASCII letters for the
language, an underscore, and two uppercase ASCII letters for the
country. Under Windows, the string can be arbitrarily long, but the
language and country are in English (all ASCII letters or spaces)
separated by an underscore. Under Unix, the result is determined by
checking the LC_ALL, LC_TYPE, and LANG
environment variables, in that order (and the result is used if the
environment variable's value starts with two lowercase ASCII letters,
an underscore, and two uppercase ASCII letters, followed by either
nothing or a period). Under Windows and Mac OS X, the result is
determined by system calls.

(system-library-subpath[variant?]) returns a relative directory
path string. This string can be used to build paths to
system-specific files. For example, when MzScheme is running under
Solaris on a Sparc architecture, the subpath is
"sparc-solaris", while the subpath for Windows on an Intel
architecture is "win32\\i386". The subpath also
distinguishes among MzScheme variants (e.g., the ``3m'' variant with
more precise garbage collection) by extending the ``normal'' variant
path with a subdirectory. If variant? is #f, then
the returned path is for the ``normal'' variant.

(version) returns an immutable string indicating the currently
executing version of MzScheme.

(banner) returns an immutable string for MzScheme's start-up
banner text (or the banner text for an embedding program, such as
MrEd). The banner string ends with a newline.

(vector-set-performance-stats!mutable-vector[thread]) sets
elements in mutable-vector to report current performance
statistics. If thread is specified, a particular set of
thread-specific statistics are reported, otherwise a different set of
global statics are reported.

For global statistics, up to 8 elements are set in the vector,
starting from the beginning. (In future versions of MzScheme,
additional elements will be set.) If mutable-vector has n
elements where n < 8, then the n elements are set to the first
n performance-statistics values. The reported statistics values are
as follows, in the order that they are set within
mutable-vector:

0: The same value as returned
by current-process-milliseconds (see section 15.1.2).

1: The same value as returned
by current-milliseconds (see section 15.1.2).

2: The same value as returned
by current-gc-milliseconds (see section 15.1.2).

3: The number of garbage collections performed since
start-up.

4: The number of thread context switches performed since
start-up.

5: The number of internal stack overflows handled since
start-up.

6: The number of threads currently scheduled for
execution (i.e., threads that are running, not suspended, and not
unscheduled due to a synchronization).

7: The number of syntax objects read from compiled code
since start-up.

For thread-specific statistics, up to 4 elements are set in the
vector:

0: #t if the thread is running, #f
otherwise (same result as thread-running?).

1: #t if the thread has terminated,
#f otherwise (same result as thread-dead?).

2: #t if the thread is currently blocked on a
synchronizable event (or sleeping for some number of milliseconds),
#f otherwise.

3: The number of bytes currently in use for the
thread's continuation.

52 The value produced for the time-zone-offset field
tends to be sensitive to the value of the "TZ"
environment variable, especially on Unix
platforms. Consult the system documentation (usually
under tzset) for details.